Theoretical studies of the mechanism of N-hydroxylation of primary aromatic amines by cytochrome P450 1A2: radicaloid or anionic?

Primary aromatic and heteroaromatic amines are notoriously known as potential mutagens and carcinogens. The major event of the mechanism of their mutagenicity is N-hydroxylation by P450 enzymes, primarily P450 1A2 (CYP1A2), which leads to the formation of nitrenium ions that covalently modify nucleobases of DNA. Energy profiles of the NH bond activation steps of two possible mechanisms of N-hydroxylation of a number of aromatic amines by CYP1A2, radicaloid and anionic, are studied by dispersion-corrected DFT calculations. The classical radicaloid mechanism is mediated by H-atom transfer to the electrophilic ferryl-oxo intermediate of the P450 catalytic cycle (called Compound I or Cpd I), whereas the alternative anionic mechanism involves proton transfer to the preceding nucleophilic ferrous-peroxo species. The key structural features of the catalytic site of human CYP1A2 revealed by X-ray crystallography are maintained in calculations. The obtained DFT reaction profiles and additional calculations that account for nondynamical electron correlation suggest that Cpd I has higher thermodynamic drive to activate aromatic amines than the ferrous-peroxo species. Nevertheless, the anionic mechanism is demonstrated to be consistent with a variety of experimental observations. Thus, energy of the proton transfer from aromatic amines to the ferrous-peroxo dianion splits aromatic amines into two classes with different mutagenicity mechanisms. Favorable or slightly unfavorable barrier-free proton transfer is inherent in compounds that undergo nitrenium ion mediated mutagenicity. Monocyclic electron-rich aromatic amines that do not follow this mutagenicity mechanism show significantly unfavorable proton transfer. Feasibility of the entire anionic mechanism is demonstrated by favorable Gibbs energy profiles of both chemical steps, NH bond activation, and NO bond formation. Taken together, results suggest that the N-hydroxylation of aromatic amines in CYP1A2 undergoes the anionic mechanism. Possible reasons for the apparent inability of Cpd I to activate aromatic amines in CYP1A2 are discussed.